A Protein Preparation Method for the High-throughput Identification of Proteins Interacting with a Nuclear Cofactor Using LC-MS/MS Analysis

Overall goal of this procedure is to identify proteins interacting with a Nuclear Cofactor using LC-MS/MS Analysis. This method can help answer key questions related to transcriptional mechanisms in the gene regulation field. The main advantage of this technique is that we can identify the proteins interacting with a nuclear cofactor.

Moreover, this method allows us to better understand the transcriptional regulatory mechanisms of the targeted nuclear factors. Harvest HEK 293 cells expressing flag-tagged ARIP4 36 hours after transvection by washing the cell sheet with ice cold PBS and collecting the cells with a cell scrapper. Transfer the cells into a 1.5 milliliter tube and then centrifuge at 8000g for two minutes at four degrees Celsius.

Aspirate the supernatant and re-suspend the cell pellet in five times the pellet volume of High Salt Buffer. Rotate the mixture for 20 minutes at four degrees Celsius. After 20 minutes of rotation, centrifuge the tube at 17, 700g for 10 minutes at four degrees Celsius.

Next, transfer the supernatants to a new two milliliter tube and add Dilution buffer to three times the volume of supernatant. Perform another cetrifugation and then transfer the supernatant to a new two milliliter tube. During the 10 minute centrifugation wash 50 microliters of anti-flag beads with PBS.

Then centrifuge the tube at 2000g for one minute at four degrees Celsius. Remove the supernatant and repeat wash steps first with 1 molar glycine hydrochloride and then with one molar tris hydrochloride. Then wash twice with Low salt buffer.

Next, mix 50 microliters of washed beads with whole-cell extracts. Gently rotate the mixture for four hours at two to four degrees Celsius. Then use a blunted tip to transfer the sample into a micro spin column and allow the solution to drip through into a 1.5 milliliter tube.

Snap freeze the flow through in liquid nitrogen and store at minus 80 degrees Celsius. Next, wash the flag beads with Low salt buffer five times. To elute, place the column into a 1.5 milliliter tube and centrifuge at 2000g for one minute to dry the beads.

Cap the bottom of the column and then add 50 microliters of 1 molar glycine hydrochloride. Gently shake the mixture for 10 minutes at room temperature. After 10 minutes, place the column into a new 1.5 milliliter tube and centrifuge at 2000g for one minute.

Neutralize the eluted solution with 10 microliters of one molar tris hydrochloride and mix well by pipetting and vortexing. Add 50 microliters of 100 millimolar ammonium bicarbonate. 10 microliters of acetonitrile, two microliters of dithiothreitol and 50 microliters of neutralized sample.

Incubate for 30 minutes at 56 degrees Celsius. After incubation, leave the sample at room temperature for 10 minutes. Then add 10 microliters of 333 millimolar iodoacetamide to the sample and incubate in the dark for 30 minutes at 37 degrees C.Add 10 microliters of trypsin to the mixture and incubate overnight at 37 degrees Celsius.

The next day transport the final product to an LC-MS/MS facility or to a third party mass spectrometry lab for analysis. This image shows silver staining of flag epitope tagged ARIP4 expressed in HEK 293 cells after immunal precipitation with a flag specific antibody and separation by electrophoresis. As a control, a mock purification was performed on HEK 293 cells that did not express exogenous proteins.

Using our method, it is possible to identify putative nuclear cofactors via their transcriptional factor of interest and further enhance our understanding of transcriptional regulatory mechanisms.